Project Summary/Abstract Immunosenescence, the age-associated decline in immunity, leads to increased susceptibility to infection and reduced vaccine efficacies. Most influenza-associated deaths in industrial countries occur among the elderly. Chronic viral infection and age-associated inflammation are known to promote immunosenescence. Immunosenescence of CD8 T cells is characterized by reduced response to immune stimuli, increased terminal differentiation, and elevated expression of co-inhibitor receptors. In contrast, T cells with stem cell-like characteristics survive decades after vaccination and exhibit superior therapeutic effects in immunotherapies. In mice chronically infected by lymphocytic choriomeningitis virus (LCMV), I have identified antiviral CD8 T cells expressing high levels of transcription factor TCF1, which resemble stem cells and can both self-renew and replenish the more terminally differentiated TCF1low CD8 T cells. Moreover, stem cell-like CD8 T cells are critical for long-lasting CD8 T cell response against chronic LCMV infection and provide better protection against re-infection. Thus, Stem cell-like CD8 T cells are ideal targets for developing improved vaccines and immunotherapies against infection in the elderly. However, aging is associated with increased inflammation. Whether age-associated inflammation influences the differentiation of stem cell-like CD8 T cells is unclear. My preliminary data suggest that upon stimulation aged CD8 T cells are more prone to terminal differentiation and exhibits elevated activation of STAT3, a transcription factor that mediates the signaling of several inflammatory cytokines. I have also found evidence that inflammatory cytokine IL-21, STAT3, and Sestrin3 are potential regulators of stem cell-like CD8 T cell differentiation. Thus, I hypothesize that age-associated inflammatory cytokine signaling mediated by STAT3 inhibits the differentiation of stem cell-like CD8 T cells in aged mice by regulating Sestrin3 expression, and can be targeted to rejuvenate aged T cell immunity. In this study, I will use my newly developed in vitro culture system and mouse chronic LCMV infection model, and employ cutting- edge transcriptomic, epigenomic, and gene editing methods to study how aging and inflammation affect the differentiation of stem cell-like CD8 T cells. The results from this study will unveil how age-related inflammatory cytokine pathways regulate the differentiation of stem cell-like CD8 T cells during aging, and facilitate the development of new therapeutic strategies aiming to rejuvenate aged T cell immunity by enhancing stem cell- like CD8 T cell differentiation. My long-term goal is to lead a research group focusing on how to harness T cells to prevent and treat infectious diseases in the elderly. The K99/R00 mechanism will provide me the training, time and resource to gain a sound foothold in the field of immunosenescence and learn new skills through getting expert guidance as well as attending courses, seminars and conferences. !